Anti-Stokes fluorescent cooling has received theoretical attention for many years. For example, in "Anti-Stokes Fluorescence As A Cooling Process," by Shaul Yatsiv, Advances In Quantum Electronics, Jay R. Singer, Editor, Columbia University Press, 1961, pages 200-213, the author describes the three-level system in which short monochromatic light pulses are used to excite a particular electronic state of an ion. Relaxation processes between close-lying excited states transfer excitation from a particular state to other states of the same ion, which may then fluoresce to the ground state. Such processes can extract heat from the sample. Excitation from an isolated ground state to the lowest excited state is of particular interest, since it excludes Stokes-shifted emission lines. Additionally, Yatsiv defines an appropriate frequency width for the incident radiation to be such that excitations of other than the lowest excited state will be excluded. However, the cooling effect is small, and practicable devices have not been described. Moreover, Yatsiv states that multiple emission and reabsorption will not appreciably affect the final results. It can be shown that if the fluorescence is permitted to pass through a significant portion of the working substance, any cooling effect will be severely degraded. Finally, Yatsiv restricts the method to gadolinium ions as the sole rare earth usable for cooling.
The first experimental study of this type of refrigeration is reported in "Optical Refrigeration In Nd-Doped Yttrium Aluminum Garnet," by T. Kushida and J. E. Geusic, Phys. Rev. Letters 21, 1172 (1968). The authors do not report a net cooling, since it is believed that heating due to absorption by small trace impurities, probably Dy.sup.3+, exceeded the cooling in the YAIG:Nd crystal. Additionally, Nd.sup.3+ is a poor choice of dopant, since the energy levels of this ion having lower energy, are too close to the ground level. Therefore, there will be a significant number of nonradiative decays which heat the working substance. Kushida et al. also state that fluorescent cooling is capable of providing an 8.degree. cooling range.
The development of fluorescent refrigeration having significant cooling effect would facilitate the widespread use of cryogenic electronic devices, both conventional and superconducting, and would greatly simplify the deployment of low-temperature, space-based instrumentation.
Accordingly, it is an object of the present invention to provide an apparatus utilizing fluorescent refrigeration for generating significant cooling.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.